The present invention relates to a heat exchanger that may be used, e.g., for the production of carbon black. Further, the invention relates to a process for the production of carbon black and a plant including said heat exchanger for the realization of the process.
Carbon black is the term used for the pulverized forms of carbon which are produced by incomplete combustion or thermic degradation of natural gas or mineral oil. Depending upon the method of production, different types of carbon black arise, namely so called channel black, furnace black and pyrolysis black (also called thermal black).
Channel black is characterized by a lower pH, a higher content of volatile constituents and fewer chain-like structures between the particles. It has the smallest particle size of all materials produced in industry and its particles are within colloidal size range. Its major field of use is as a reinforcement material in rubber, where it improves both the wear resistance and the oil resistance of the rubber.
Thermal black consists of relatively coarser particles and is primarily used as a pigment. Furnace black, which has been produced from natural gas, has a medium size, while the furnace black produced from oil may occur within a broad range of controlled particle sizes and is particularly suitable for reinforcing synthetic rubber. Furnace black is by far the most important form of carbon black and is used to a considerably larger extent than the other two. Also the present invention relates specifically to this type of carbon black, which in the present application is referred to simply as just "carbon black".
Carbon black is commercialized in the form of a powder, or pellets or paste. The powder is kept in multi-walled paper bags or in lined barrels. Carbon black is used as an additive in rubber tries and other wear resistant rubber products. In plastics it is used as a reinforcing agent, as an opacifier, as a means for increasing the electrical conductivity and for absorbing ultraviolet light. Further, carbon black is used for instance in duplicating carbon, in ink ribbons for typewriters, in colour pigments and for influencing the weather.
A plant for the production of carbon black (i.e., of the type furnace black) depicted in FIGS. 1 and 2 is conventional, except for the support base 15 in FIG. 2. Incoming combustion air flows through a tube conduit or inlet 1 into the upper part of a tube heat exchanger 2, in which it is preheated before being combusted with oil in the burner 9 and the combustion reactor 3. The preheated air is passed into the combustion chamber via a conduit 5. Oil is added to said reactor via a tube conduit 4. The amount of air corresponds to about 50% of the stoichiometric amount of oxygen gas required for a complete combustion of the oil, whereby carbon black is formed. It is also possible to add water into the reactor 3, which has an impact on the quality of the final product. The mixture of suspended carbon black in the consumed combustion air is led away from the top of the heat exchanger via a conduit 6, through a normally water-cooled cooler 7 to a filter arrangement 8, conventionally equipped with textile bag filters. In this filter arrangement the carbon black is completely filtered off from the gas flow, which is then passed out through a nonreturn valve 1G for further purification in a plant 11 before it is exhausted into the ambient air via a chimney 12.
The construction of the heat exchanger 2 may be more clearly seen in FIG. 2. The heat exchanger is of the tube type, with a plurality of substantially vertical tubes 13. Inside these tubes rise the hot gases from the combustion, whereby they are cooled by the air that enters via the inlet 1 and passes outside the tubes 13 downwards towards the outlet 5, within a shell 14. The temperature at the inlet 1 of the tubes 13 may be about 1000.degree. C. and the combustion air passing along the exterior of the tubes may be preheated to about 800.degree. C. These conditions result in utmost severe stresses for the materials in the heat exchanger. The part of the heat exchanger that is submitted to the highest mechanical stress is the lower part of the shell, where the metal temperature may amount to about 900.degree. C.
A measured temperature profile in the shell 14 is shown in the diagram in FIG. 4. At a simultaneous inner pressure of about 1 bar and a shell diameter of about 2000 mm, and a tower height of about 13 m, this implies that very thick metal walls have to be used since the strength of the material at these temperatures only is in the order of magnitude 5N/mm.sup.2. Instability may easily arise in the shell at downtimes, whereby the risk of buckling is imminent. A number of shut-downs because of bucklings have also occurred, with considerable costs as a consequence. Thus, there are strong reasons for wanting to decrease the temperature of the shell, thereby working under conditions where the strength in the shell is sufficiently high to avoid problems relative to instability.
An undesired process phenomenon that arises in the heat exchanger is that the tubes 13 may be fouled by carbon. Thus, the incoming air of ambient temperature or somewhat higher, cools the tube wall and the burnt gases, which are in equilibrium at the inlets of the tubes 13 and will therefore be excessively cooled on the inside of the tube, whereby carbon may be formed according to the Boudoir reaction 2CO.fwdarw.C+CO.sub.2. Together with the carbon particles in the gas, the thus formed carbon contributes to the fouling. One way of reducing the tendency of fouling would be to diminish the degree of cooling of the gas, i.e., to operate with warmer tube walls. This could be achieved by preheating the air that flows into the conduit 1. However, then the lower part of the shell would become even hotter and the stress upon this part would thus be further accentuated.
Hence, a primary object of the present invention is to reduce the thermal stress upon the shell of a tube heat exchanger, in particular during the production of carbon black.
A second object of the present invention is to prevent foulings by carbon within the heat exchanger tubes.
Further objects of the present invention are to extend the useful life of the heat exchanger during the production of carbon black and to minimize the number of running disturbances in said exchanger.
Still another object of the present invention is to improve the efficiency of the heat exchanger.